Recently, with increasing demand for portable electronic devices such as mobile phones, there is a concomitant need for high performance semiconductor devices to power such portable equipment. In connection with rapid progress in fine-process technology in semiconductor processing, it has become possible to manufacture extremely fine, dense internal circuitry. Accordingly, a lower operating voltage must be supplied to the circuits manufactured with the fine process, especially with the central processing unit (CPU). For example, the CPU circuits of previous generations are designed to be operated at 5 v±5%. By contrast, the CPU circuits of the current generation are designed to be operated at 3.3 v±5%. Further, the CPU circuits of the coming generation will have to operate at 2.5 v±5%.
If all peripheral circuits employed together with such CPU are developed to operate at the same operating voltage as that for the CPU, it needs relatively long term to develop the system. Further, the advantages of mass production are reduced, resulting in a cost penalty.
If peripheral devices which can operate at both 5 v±5% and 3.3 v±5% are developed, it is possible to shorten development time and achieve cost reduction owing to the advantages of mass production.
Further, when a laser diode drive circuit is included in the semiconductor device, the laser diode drive circuit is requested to operate under a wide supply voltage because the drive voltage of the laser diode differs according to the emission wavelength of the laser diode.
To satisfy such demand, JP-2003-78202-A describes a laser diode drive circuit that includes a plurality of supply voltage sources to generate a plurality of voltages to drive a plurality of laser diodes each of which needs a different drive voltage, a supply voltage source exchange unit, a current amplifier, and a laser diode exchange unit. The supply voltage source exchange unit switches the supply voltage source to supply the appropriate drive voltage to the laser diode to be driven. The current amplifier supplies the current to the laser diode. The laser diode exchange unit switches the laser diode to obtain the supply current to the laser diode. Accordingly, the circuit elements of the current amplifier are provided to each of the respective laser diodes independently.
FIG. 1 is a circuit diagram of a known semiconductor device. The semiconductor device 100 includes an internal circuit 101 and a reset circuit 102. The internal circuit 101 performs a predetermined function. The reset circuit 102 generates a reset signal RES that puts the internal circuit 101 in a predetermined reset state. The electronic circuit in the semiconductor device 100 is designed to operate under a predetermined power supply voltage range, for example, 5 v±5%, and 3.3 v±5%. Accordingly, if the power supply voltage exceeds the power supply voltage range for the design basis, it is not possible for the circuit to provide maximum performance. Further, if a high voltage much higher than the predetermined power supply voltage range is applied, the semiconductor device might fail.
If the circuit is designed to operate at a low operating voltage with high voltage transistors, it may be able to operate over a wide supply voltage range between a low voltage region and a high voltage region. However, such circuit does not operate optimally under both high voltage conditions and low voltage conditions.
For example, FIG. 2 is a circuit diagram of a known supply voltage drop detection circuit provided in the internal circuit to protect the semiconductor device and display an alarm when the supply voltage drops. The supply voltage drop detection circuit of FIG. 2 includes a comparator 121, bleeder resistors R121 and R122, and a reference voltage source 123 that generates a predetermined reference voltage VrB. In the supply voltage drop detection circuit of FIG. 2, a supply voltage VA or VB is input to a power supply voltage terminal Vdd. The supply voltage VA or VB is divided by the bleeder resistors R121 and R122 to input a divided voltage Vin to the comparator 121. The comparator 121 compares the divided voltage Vin and the reference voltage VrB. If the divided voltage Vin is higher than the reference voltage VrB, the comparator 121 output is high. If the divided voltage Vin is equal to or lower than the reference voltage VrB, the comparator 121 output is low. Thus, the output signal of the comparator 121 is a voltage drop detection signal.
FIGS. 3 and 4 are timing charts representing an example operation of the circuit of FIG. 2. FIG. 3 represents a case in which the low power supply voltage VB is input to the power supply voltage terminal Vdd. FIG. 4 represents a case in which the high power supply voltage VA is input to the power supply voltage terminal Vdd. A reference voltage VrA is a voltage value of the divided voltage Vin to detect a voltage drop when the power supply voltage is VA. A voltage VrB is a voltage value of the divided voltage Vin to detect a voltage drop when the power supply voltage is VB.
In FIG. 3, when the power supply voltage VB is decreased and the divided voltage Vin becomes lower than the reference voltage VrB, the comparator 121 outputs a low level signal as the voltage drop detection signal.
However, in FIG. 4, when the power supply voltage VA is decreased and the divided voltage Vin becomes lower than the reference voltage VrA, the comparator 121 does not invert the output signal, and does not output the voltage drop detection signal until the divided voltage Vin drops to the reference voltage VrB. This is because the circuit is set so that the voltage VrB is used as the reference voltage and optimized for a case of the low power supply voltage VB.
Conversely, if the reference voltage is set to the voltage VrA, the voltage drop detection signal continues to be output when the power supply voltage VB is used. Accordingly, it is not possible to set the voltage VrA as the reference voltage. Thus, when the supply voltage is the high power supply voltage VA, it is not possible to detect at a desired voltage, resulting in degeneration in accuracy of detection of the supply voltage drop.
FIG. 5 is a circuit diagram of a known terminal voltage detection circuit which outputs a terminal voltage detection signal when a voltage input to an external terminal T1 of the semiconductor provided in the internal circuit 101 becomes equal to or higher than a predetermined voltage. FIG. 6 is a timing chart representing an example operation of the circuit of FIG. 5. In FIG. 6, Vin+ is a voltage value of the divided voltage Vin when the power supply voltage VA or VB swings fully towards the upper limit. In FIG. 6, Vin− is a voltage value of the divided voltage Vin when the power supply voltage VA or VB swings fully towards the lower limit. In the terminal voltage detection circuit of FIG. 5, a comparator 122 compares a divided voltage Vin obtained by dividing the supply voltage VA or VB by the bleeder resistors R123 and R124 with a divided voltage VT1 obtained by dividing the voltage input to an external terminal T1 by the bleeder resistors R125 and R126. Accordingly, the output signal of the comparator 122 is a terminal voltage detection signal.
When the voltage VT1 is higher than the divided voltage Vin, a terminal voltage detection signal becomes high. When the voltage VT1 is lower than the divided voltage Vin, the terminal voltage detection signal becomes low.
However, the power supply voltage VA or VB varies within a predetermined voltage range. Accordingly, a pulse width of the terminal voltage detection signal changes significantly between a state in which the divided voltage Vin is Vin+ and a state in which the divided voltage Vin is Vin−. Accordingly, it may not be possible to detect the voltage at the external terminal T1 accurately.
FIG. 7 is a circuit diagram of known bias circuit which is used, for example, in an amplifier circuit provided in the internal circuit 101. In FIG. 7, a PMOS transistor M132 is provided and a predetermined bias voltage is input to a gate of the PMOS transistor M132 to eliminate a fluctuation of the drain voltage of the PMOS transistor M131 due to a fluctuation of the supply voltage input to the power supply terminal Vdd. However, the bias voltage is set to an intermediate voltage to operate both under the power supply voltages VA and VB. Consequently, the bias voltage can not be the best bias condition for each power supply voltage VA and VB.
FIG. 8 represents a switching transistor and drive circuit. As shown in FIG. 8, a switching transistor is formed of a lot of small size transistors connected in parallel, each transistor has a predetermined general size.
More specifically, in FIG. 8, the switch transistor is formed of PMOS transistors M141 through M144. An output terminal of a buffer circuit 141 having a large driving power is connected to each gate of the PMOS transistors M141 through M144. A buffer circuit 142 having a small driving power is connected to an input terminal of the buffer circuit 141.
When the power supply voltage is low, the operational voltage range is narrow. For this reason, it is necessary to make the switching transistor large in size. In the prior circuit, the number of transistors connected in parallel is increased to obtain a large switching transistor. However, if the number of transistors is increased, it may exceed a necessary performance, i.e., over-specification, and a parasitic capacitance of the switching transistor is increased. A switching transistor that switches at high speed generates a large noise when the power supply voltage is high and the parasitic capacitance is large. Accordingly, if the number of transistors is increased to fit for a condition when the power supply voltage is low, a large noise may be generated when the circuit operates under a high power supply voltage.
Further, in a semiconductor that includes a known laser drive circuit in the internal circuit 101, as described previously each laser diode needs a different drive voltage. Accordingly, an appropriate laser drive circuit is connected to each laser diode by switching the connection to the laser drive circuits to supply power to laser drive. Thus, it is necessary to prepare a circuit dedicated to extra laser diodes that are not used, resulting in a cost penalty.